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I am studying how animals (including the human beings) can perceive distances thanks to their eyes and their ears. I am focusing on the fact that they always go in pairs: two eyes, two ears, etc.

About the sight, I think our eyes use the parallax. But is this enough? Is this the only way we can perceive distances with them?

About the sounds, I think it is merely by interferometry, but I am not convinced, because the distances between our ears and the wavelength are not always comparable to one another.

What do you think about it? Thanks in advance,

Isaac

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If it was the parallax you wouldnt be able to judge distances with 1 eye closed. Anyways it your brain that perceive distances, there is no information of distance in the 2D picture your eye receives. You can look at a photograph and still judge distances. –  user1708 Apr 13 '11 at 9:40
    
If you see something which is blurry, you will think its farther away, similarly if you see a human shape, you will think its a human and judge size based on your experience –  user1708 Apr 13 '11 at 10:00
    
By parallax, I mean that when your eyes focus on a precise point, they make some angles that can be used easily to calculate its short distance quite precisely. Then, for too big distances, it becomes extremely unprecise. Of course, as you say, if you know the size of an object, it is easier to determine its distance; that's evidence. My question is partially about the eventual other ways to determine distances with our eyes. –  Isaac Apr 13 '11 at 12:05
    
@Feynstein, there is a simple test that shows we can't judge distances as well with one eye closed. Hold out your hands in front of you and point your index fingers at each other. Now bring your fingers together. You will miss more often than not... –  Nic Apr 13 '11 at 12:09
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FWIW, I can do the fingers thing with my eyes closed, so deduce my hearing is especially acute. –  Mark Eichenlaub Apr 14 '11 at 6:32
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2 Answers

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With vision depth is determined by parallax. This largely works for objects out to 100m or less. Depth beyond that distance is assessed by familiarity with these objects, say large mountains or vistas, and experience with them.

Depth with hearing is determined by two means. A tone which is perceived louder in one ear than the other is usually perceived as having a direction based on that. For lower frequency sounds phase differences between left and right auditory perception can be used by the brain to detect angular direction relative to the facial direction.

Some species of whales are thought to generate a mental map of the ocean, where a blue whale in the Atlantic maps the outlay of the ocean bottom and coasts through echo location. Clearly this indicates a huge amount of neural processing which uses these acoustical data. Dogs generate maps of their immediate world through olfactory means. Life forms generate maps of their world by a variety of means.

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With regards to hearing, it can be hard to determine whether a sound is coming from directly in front or directly behind as the sound reaches each ear simultaneously. A feature I noticed frequently while working in a call centre when a phone rang. –  Nic Apr 13 '11 at 12:12
    
It might be prudent to add that depth in vision is assessed in many more ways than mentioned in this answer, although they don't rely on binocular vision. There is depth from shading and lighting, atmospheric haze etc. –  BjornW Apr 13 '11 at 12:29
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Adding to Lawrence's answer, apart from the monocular cues like depth from shading, lighting, haze, geometrical structure and familiarity of objects, you also have binocular cues from the eye motor convergence system (eyes point at the spot you are looking at in 3d space) as well as the feedback from the lens focusing system.

All of these systems work together when you look at objects in the real world, but in for example 3d movies and tvs only some are active, which can cause various sorts of dizziness or headache among viewers.

Interestingly viewing plain old 2d photos of 3d objects/worlds does not infer this sickness at all..

With regards to hearing, apart from loudness and phase the auditory system also uses timing of onset, this obviously relates to phase discrimination depending on the audio frequency and size of the head. Furthermore, the spectral filtering of the signals at both ears is shaped by the asymmetrical shape of the outer ears, this allows the auditory system to gauge sound localization in front of, behind, above and below. Monaural cues include, as with vision, familiarity of certain sounds or speakers and how these sounds degrade by passing through air.

Several hundred million years of evolution has been pretty good at extracting every bit of useful information from all our senses :)

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I think the dizziness / headaches 3D systems cause is by the 3D effect not giving a correct parallax or making things looking distorted, not from the lack of other cues. –  Mr X Apr 13 '11 at 15:12
    
@Jeremy: yes, what I meant was that when the other cues are "inactive", the cues contradict each other, for example the stereo disparity is correct due to the 3d glasses, but the eye convergence is on the screen, and the focus is even worse, it is up to the director of photography. –  BjornW Apr 13 '11 at 16:06
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